Multifrequency electromagnetic geophysical tools for evaluating the hydrologic conditions and performance of evapotranspiration barriers

Abstract

Surface barriers are designed to isolate subsurface contaminants for 1000 years or longer, functionally limiting water infiltration and removing the driving force for contaminant transport to groundwater. Cost-effective monitoring is challenging because of the long design life for surface barriers, spatial limitations and finite lifetime of in situ sensors, and performance metrics related to drainage. Hence, ground-penetrating radar (GPR) and electromagnetic induction (EMI) tools were evaluated for use in performance monitoring of surface barriers. GPR and EMI were used to non-invasively interrogate the Prototype Hanford Barrier (PHB), an evapotranspiration-capillary break barrier established in 1994 at the Hanford Site, in southeastern Washington State. Both geophysical methods were evaluated for providing indirect estimates of subsurface moisture content conditions that were compared to point scale measurements from borehole neutron logs. Surveys were performed during characteristically wet and dry periods to observe a range of hydrologic states of the barrier soil. Although EMI surveys were expected to show seasonal changes associated with changes in the bulk conductivity of the barrier soil layers, the effectiveness of the method was limited by the effects of metallic infrastructure embedded in the barrier. GPR estimates of volumetric water content were typically within 2–3% of the highest water contents from neutron probe measurements for both wet and dry periods, providing reasonable estimates of water content. Given that PHB monitoring data over the past 25 years has demonstrated its success in limiting deep drainage, GPR was found to be a cost-effective method for demonstrating continued barrier performance, with a greater capacity to quantify moisture content distributions over much larger areas relative to point measurements.